Actinopterygian and chondrichthyan palaeocommunity responses to early Cenozoic global warming

Bird, Hannah (2025). Actinopterygian and chondrichthyan palaeocommunity responses to early Cenozoic global warming. University of Birmingham. Ph.D.

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Abstract

Climate change in the geological past can be a valuable tool for assessing the context of current and future global warming, with numerous warming events punctuating the early Paleogene (~48 to 66 Ma) greenhouse world considered partially analogous to today’s anthropogenic climate change. The Palaeocene-Eocene Thermal Maximum (PETM, ~56 Ma), Early Eocene Climate Optimum (EECO, ~49 to 53 Ma) and Middle Eocene Climate Optimum (MECO, ~40.1 to 40.5 Ma) are three key intervals of environmental change, during which atmospheric carbon dioxide peaked at levels akin to predictions by the Intergovernmental Panel on Climate Change for 2100 under worst-case emissions scenarios. Consequently, the interplay of the atmosphere and ocean in response to global warming and environmental change is important to understand the ecological and biological impacts, with inferences on how this may translate to the modern day and the future.

Palaeontology is invaluable for exploring biotic responses to such climate events as fossils can indicate the abundance and diversity of palaeocommunities through time. To understand the response of the marine realm to early Paleogene climate perturbations, actinopterygian (bony fish) and chondrichthyan (shark) fossils are important as they comprised over half of all marine vertebrate diversity at this time, and fish radiated due to expansion of ecological niches after the end-Cretaceous mass extinction. Whilst complete articulated body fossils of fish and sharks are relatively rare, microfossils from them are considerably more abundant, with the possibility to generate higher temporal and spatial resolution records. Important microfossils are otoliths (calcareous components in vertebrate inner ears), teeth and denticles (tooth-like scales external of cartilaginous sharks), collectively termed ichthyoliths, which enable reconstruction of fish and shark productivity and diversity.

Museum collections provide an important but often underutilised resource for palaeobiodiversity studies, partly due to poor data accessibility (particularly digitally) but also because the value of such collections in large-scale analyses is highly dependent upon the quality of the information associated with them. Here, the potential of the largest public early Eocene (~48 to 56 Ma) actinopterygian otolith collection in the United Kingdom (housed in the Natural History Museum, London) is evaluated to reconstruct the shallow marine bony fish palaeocommunity from southern England and test if there is a true radiation post-Cretaceous or if sampling and museum curatorial biases are impacting the signal.

Turning to the response of deeper marine fish and shark palaeocommunities to climate change, fish teeth and shark denticles extracted from Indian and South Atlantic Ocean cores are assessed to understand productivity and diversity responses to short-lived PETM warming, long-term EECO warming and intermediate MECO warming. Whilst fish productivity is often elevated during warming intervals, diversity responses fluctuate between these three climate events. Ultimately, the rate of environmental change over which the onset and termination of each event occurs is a major determinant of the ability of fish, in particular, to adapt to the ambient environmental perturbations. As such, fish turnover associated with the rapid onset of the PETM and termination of the MECO highlight the potential ‘short-term’ instability of fish communities responding to modern climate change, but the ‘longer-term’ ability to equilibrate for a thriving marine ecosystem in the future.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):

Supervisor(s)	Email	ORCID
Edgar, Kirsty	UNSPECIFIED	UNSPECIFIED
Sansom, Ivan	UNSPECIFIED	UNSPECIFIED
Butler, Richard	UNSPECIFIED	UNSPECIFIED
Sibert, Elizabeth	UNSPECIFIED	UNSPECIFIED
Twitchett, Richard	UNSPECIFIED	UNSPECIFIED